Abstract: The mesostructure of Mn_1 – xFe_x Ge transition-metal monogermanides is studied by small-angle neutron scattering (SANS) and ultra-SANS in a wide concentration range of x = 0.0–1.0.It is shown that the main contribution to the scattering intensity for all concentrations x is made by scattering at crystallites with sharp boundaries and sizes of 1–10 μm, which is described by the squared Lorentzian function. An additional contribution to the scattering intensity as a result of scattering at an ensemble of defects is found as well, which is characteristic of manganese-rich samples. This contribution is well fitted by the power function Q^–n with the exponent n = 3. The complementary scattering typical of iron-rich samples is described by an exponential function and also seems to be a part of scattering at sharp-boundary crystallites.